Quantifying nematic order in evaporation-driven self-assembly of Halloysite nanotubes: Nematic islands and critical aspect ratio

TL;DR

This study quantifies how evaporation-driven self-assembly of polydisperse Halloysite nanotubes leads to inhomogeneous nematic ordering, phase coexistence, and aspect ratio-dependent sorting, revealing new insights into controlling nanostructure formation.

Contribution

It introduces a detailed quantification of nematic order and phase transitions in HNT self-assembly, highlighting the role of aspect ratio in ordering and structure formation.

Findings

Nematic order parameter $S$ correlates with local aspect ratio $L/D$.

Nematic islands form at the isotropic-nematic transition.

Order occurs only for rods with $L/D extgreater 6.5 \

Abstract

Halloysite nanotubes (HNTs) are naturally occurring clay minerals found in Earth's crust that typically exist in the form of high aspect-ratio nanometers-long rods. Here, we investigate the evaporation-driven self-assembly process of HNTs and show that a highly polydisperse collection of HNTs self-sort into a spatially inhomogeneous structure, displaying a systematic variation in the resulting nematic order. Through detailed quantification using nematic order parameter $S$ and nematic correlation functions, we show the existence of well-defined isotropic-nematic transitions in the emerging structures. We also show that the onset of these transitions gives rise to the formation of nematic islands - phase coexisting ordered nematic domains surrounded by isotropic phase - which grow in size with $S$. Detailed image analysis indicates a strong correlation between local $S$ and the local aspect ratio, $L/D$, with nematic order possible only for rods with $L/D \ge 6.5 \pm 1$. Finally, we conclude that observed phenomena directly result from aspect ratio-based sorting in our system. Altogether, our results provide a unique method of tuning the local microscopic structure in self-assembled HNTs using $L/D$ as an external parameter.